1. Field of the Invention
The invention relates to an electric power steering system.
2. Discussion of the Related Art
In an electric power steering system in which an output from an electric motor is transmitted to a steering mechanism via a reduction mechanism, a worm reduction mechanism may be used as the reduction mechanism (for example, refer to paragraphs 0035, 0037, and 0038 of Japanese Patent Application Publication No. 2004-301265). A worm shaft of the worm reduction mechanism is connected to a rotation shaft of the electric motor. Both ends of the worm shaft are supported by a housing via bearings. Among these bearings, the bearing that supports one end portion of the worm shaft, to which the electric motor is connected, is a ball bearing. The inner ring of the ball bearing is allowed to oscillate relative to the outer ring. More specifically, the curvature radii of the raceway grooves of the inner and outer rings of the ball bearing are made large enough to allow the inner ring to oscillate relative to the outer ring. The other end portion of the worm shaft is urged toward the worm wheel side by an elastic member.
With the structure described above, the worm shaft is tilted about the one end portion toward the worm wheel side, thereby eliminating the backlash at a meshing portion where the worm shaft and the worm wheel mesh with each other. Further, when an axial load acting in the axial direction of the worm shaft is applied from the worm wheel to the worm shaft, the inner and outer rings of the ball bearing elastically deflect, allowing the worm shaft to slightly move in the axial direction. With this structure, the worm shaft is axially movable under a force transmitted from a steering wheel to the worm shaft via the worm wheel when the electric motor is not driven, for example, at the moment at which turning of the steering wheel is started. Accordingly, a resistive force that the worm shaft receives from the electric motor when the electric motor is not driven is smaller. As a result, it is possible to reduce a steering operation load that is placed on the driver when the electric motor is not driven.
Usually, there are internal clearances in a ball bearing. If the internal clearances are too large, when a worm shaft moves in the axial direction, an inner ring of the bearing moves in the axial direction together with the worm shaft. Then, balls of the bearing hit the inner and outer rings, and consequently contact noise is generated. On the other hand, if the internal clearances in the bearing are too small, the movement of the worm shaft is hindered excessively. More specifically, if the internal clearances in the bearing are too small, it is not possible to sufficiently move an end portion of the worm shaft, which is distant from a motor, toward a worm wheel. Thus, it is not possible to eliminate the backlash that is generated between the worm shaft and the worm wheel as the teeth of the worm shaft wear. Therefore, contact noise, due to the backlash, occurs when the worm shaft and the worm wheel hit each other.
Accordingly, the axial internal clearances in a ball bearing need to be set to appropriate values that are neither too large nor too small, and therefore strict dimensional control over the ball bearing is required. This results in an increase in the manufacturing cost. Japanese Patent Application Publication No. 2004-301265 describes that the internal clearances in a ball bearing are made “negative clearances” by press-fitting an inner ring onto a worm shaft such that the inner ring is pressed toward an outer ring. It is considered that, with this structure, it is considered that by setting the internal clearances in the bearing to be large when the bearing is not mounted on the worm shaft, an amount by which the worm shaft is allowed to move is made sufficiently large, while suppressing generation of the contact noise that would occur due to internal clearances when the balls and the inner and outer rings hit each other.
With the structure described in Japanese Patent Application Publication No. 2004-301265, however, it is necessary to accurately match the outer diameter of the worm shaft, which is an elongated shaft, with the inner diameter of the inner ring of the ball bearing to allow the worm shaft to press the inner ring in the radial direction by an appropriate amount. That is, it is necessary to execute considerably strict dimensional control to achieve an appropriate fitting-interference between the worm shaft and the inner ring of the ball bearing. Therefore, a lot of time and efforts are required to fit the inner ring of the ball bearing onto the worm shaft.
In the structure described in Japanese Patent Application Publication No. 2004-301265, because the axial movement of the worm shaft is allowed using the deflection of the steel inner and outer rings of the ball bearing, the amount by which the worm shaft is allowed to move in the axial direction is considerably small. For this reason, the worm shaft is able to move by only a small amount in the axial direction when an axial load is applied from a steering operation member to the worm shaft via the worm wheel, and therefore a resistive force from the electric motor when it is not driven cannot be sufficiently reduced. As a result, a large reaction force acts on the steering operation member via the worm shaft and the worm wheel, and thus the steering operation load placed on the driver cannot be sufficiently reduced.